The present invention relates to a circuit for the reading of linear arrays of photodetectors. It can be applied especially to high-performance thermal imaging devices using optomechanical scanning of the image perpendicularly to a linear array of photodetectors.
In these imaging devices, the scanning is done by an appropriate optomechanical system comprising in particular a mobile mirror and an optical assembly for the formation and projection of an image of the observed scene on the detection linear array.
Conventionally, in the direction perpendicular to the scanning, the linear array comprises a number N of detection channels enabling the parallel reading of N instantaneous fields. In order to improve the sensitivity of the imaging devices, each detection channel comprises a determined number N.sub.d of "sensors" aligned in the scanning direction, such that each point or pixel of the image-taking field is analyzed successively by N.sub.d sensors. The improvement of the sensitivity is obtained by the recomputation, in phase, of the electrical signals delivered by each of the sensors of one and the same channel and then by the adding together of these signals. Since the noise is added quadratically, the addition of the signals given by the N.sub.d sensors enables the signal-to-noise ratio to be improved by the factor .sqroot.N.sub.d . This processing of the signals is known in specialized literature as "time delay and integration" or TDI.
In the known embodiments, with each detection element there is associated a circuit for the injection and integration of the photo-charges generated by the photosensitive cells proportionally to the illumination received. At the end of the integration period, the state of the integrator is read either by means of a shift register comprising as many lateral inputs as the detection channel or, in the CMOS technology structures, by a summation circuit comprising switched-capacitance operational amplifiers.
Since the number of information elements during processing, at a given point in time, is set by the number of detection channels "N", the number of sensors per channel N.sub.d and the ratio between the pitch P.sub.d of the sensors and the sampling pitch P.sub.e of the image in the scanning direction and since each of these information elements is assigned to a Time Delay and Integration (TDI) operator comprising N.multidot.N.sub.d .multidot.P.sub.d /P.sub.e summation devices, the dimensions of the linear arrays are soon limited firstly by the space requirement of the read circuit and secondly by the dissipated power. For example, if N=1000, N.sub.d =10 and P.sub.d /P.sub.c =3, 30,000 TDI summation devices are needed.
Furthermore, the low frequency noise, the limited gain, the gain variations and offset variations of the operational amplifiers, combined with the fluctuations of the values of the summation capacities, lead to the superimposing, on the restituted video signal, of an additional noise that is difficult and costly to correct.